WASHINGTON (CNN) -- Investigators are trying to determine whether former CIA Director David Petraeus used any of the resources of the agency he once led to carry out an extramarital affair with his biographer, a U.S. official told CNN.

The news of the internal investigation came on the eve of Petraeus' scheduled closed-door testimony Friday before the U.S. House and Senate intelligence committees over the September attack on the U.S. Consulate in Benghazi, Libya, that left four Americans dead.

Petraeus will probably face questions about the timing of his resignation. It occurred just as congressional committees convened hearings on the attack, which has been the focus of Republican claims that President Barack Obama's administration either misled or lied about the events surrounding it.

Among other things, the CIA is investigating Petraeus' "general conduct," according to an official with knowledge of the investigation who spoke on condition of anonymity.

The CIA confirmed it was reviewing Petraeus' performance but did not characterize the nature of its investigation.

"At the CIA, we are constantly reviewing our performance. If there are lessons to be learned from this case, we'll use them to improve," CIA spokesman Preston Golson said.

"But we're not getting ahead of ourselves; an investigation is exploratory and doesn't presuppose any particular outcome."

Petraeus told Kyra Phillips, from CNN's sister network HLN, that his resignation was not linked to the Benghazi attack and that he never passed classified information to the woman at the center of the scandal over his affair.

"In our first conversation," Phillips said Thursday, Petraeus "told me he had engaged in something dishonorable. He sought to do the honorable thing in response -- and that was to come forward. He was very clear that he screwed up terribly ... even felt fortunate to have a wife who is far better than he deserves."

Phillips, who has interviewed Petraeus in war zones, initially reached out to the retired Army general to express shock about the news of the affair, which derailed a highly decorated career in the military and CIA.

Among other things, Petraeus said he had not talked with Paula Broadwell, his biographer and the woman with whom he had an affair, since word of the affair broke.

"He insisted to me that he has never passed classified information to Paula Broadwell," Phillips said. "He said this has nothing to do with Benghazi, and he wants to testify. He will testify."

Broadwell spent a year with Petraeus in Afghanistan as research for her book, "All In: The Education of David Petraeus."

The FBI uncovered the affair while investigating e-mails that Broadwell allegedly sent to a Petraeus family friend, Jill Kelley, according to administration officials and friends of Petraeus.

The e-mails, according to those familiar with the investigation, questioned Kelley's friendship with U.S. Marine Gen. John Allen.

The investigation into the e-mails connected to Allen and Kelley led to the discovery of the affair between Petraeus, 60, and Broadwell, 40.

Petraeus said the extramarital relationship ended months ago. But it continues to make waves in Washington, sparking collateral investigations.

The pending nomination of Allen as NATO's supreme allied commander has been put on hold, until the nature of his e-mail exchange and relationship with Kelley has been clarified.

Reports by officials familiar with the e-mail messages between Kelley and Allen give conflicting impressions about their potentially inappropriate nature -- with one U.S. official saying "John Allen would be very embarrassed by them" and a senior official close to the general saying they don't point to a sexual or romantic relationship.

That Allen remains in command in Afghanistan suggests that there is no criminal issue, a U.S. official told CNN. But the official said the Defense Department's inspector general could still find evidence of criminal conduct.

"I am not aware of any others that could be involved in this issue at the present time," Defense Secretary Leon Panetta told reporters.

"Obviously, as this matter continues to be investigated both on Capital Hill and by the inspector general, I'm sure that we'll have to wait and see what additional factors are brought to our attention."

The FBI investigation was triggered by Frederick Humphries, a veteran agent for the Tampa FBI field office, who is a friend of Kelley, said Lawrence Berger, general counsel for the Federal Law Enforcement Officers Association.

Kelley, who was a volunteer at MacDill Air Force Base in Florida, contacted Humphries about anonymous e-mails that questioned her character, a U.S. official and a source close to Kelley said.

Humphries took the information in the e-mails to the "appropriate components," Berger said. "He reported it to the proper channels and had no further part in the case."

Humphries is now the subject of an FBI investigation into whether he followed proper procedures in speaking to members of Congress about his concerns about how the FBI was handling Kelley's case.